Three families: Phocidae, Otariidae, Odobenidae
Worldwide there are about nineteen seal species, with a combined population of more than thirty million animals. Biologists arrange them in three families within the superorder Pinnipedia (the marine carnivores): the true seals (Phocidae), the eared seals (Otariidae) and the walrus (Odobenidae).
True seals lack external ear flaps — only an ear opening is visible — and their hind flippers are permanently rotated backwards, so they can only shuffle and inch their way along on land. Under water, by contrast, they are extremely efficient; their entire propulsion comes from the hind flippers. Eared seals, with visible ear flaps and rotating hind flippers, can walk on all fours on land. The walrus is a family of its own with its characteristic tusks and specialised diet of shellfish. Both Dutch species — the common seal and the grey seal — belong to the Phocidae.
True seals: 18 species
The Phocidae include eighteen species, divided into two subfamilies (Phocinae in the northern hemisphere, Monachinae in the southern hemisphere plus the monk seals).
Northern hemisphere — our region
- Common seal (Phoca vitulina)Coastal waters of the northern hemisphere — NL: yes — LC
- Grey seal (Halichoerus grypus)NW Atlantic coast, NW Europe — NL: yes — LC
- Ringed seal (Pusa hispida)Arctic sea ice — LC, locally vulnerable
- Hooded seal (Cystophora cristata)North Atlantic pack ice — VU
- Bearded seal (Erignathus barbatus)Arctic coastal seas — LC
- Harp seal (Pagophilus groenlandicus)North Atlantic pack ice — LC
- Ribbon seal (Histriophoca fasciata)Bering and Okhotsk Seas — LC
- Caspian seal (Pusa caspica)Caspian Sea (fresh/brackish) — EN
- Baikal seal (Pusa sibirica)Lake Baikal — pure freshwater — LC
- Ladoga ringed seal (Pusa hispida ladogensis)Lake Ladoga (Russia) — subspecies, vulnerable
- Saimaa ringed seal (Pusa hispida saimensis)Lake Saimaa (Finland) — subspecies, EN
The common seal is by far the most widespread species: it occurs in almost every temperate to subarctic coastal sea of the northern hemisphere, from the Waddenzee to Hokkaido and from Maine to California. The grey seal is far more limited: only the north-western Atlantic coast of Europe (British Isles, Waddenzee, Baltic, Iceland) and the Canadian part of Atlantic North America. The freshwater seals deserve a special mention: the Baikal seal in Siberia and the Saimaa ringed seal in Finland live exclusively in a single lake, having become isolated there thousands of years ago after the last ice age.
Monk seals
- Mediterranean monk seal (Monachus monachus)Mediterranean, NW Africa — EN, ~700 animals
- Hawaiian monk seal (Neomonachus schauinslandi)Hawaiian archipelago — EN, ~1,500 animals
- Caribbean monk seal (Neomonachus tropicalis)Caribbean Sea — extinct (1952)
The monk seals are the only seals that live in genuinely warm seas. They are among the most endangered mammals in the world. The Caribbean monk seal was last reliably seen in 1952 and was officially declared extinct in 2008 — making it the only seal species that humans have driven to extinction in modern times.
Southern hemisphere
- Weddell seal (Leptonychotes weddellii)Antarctica, fast pack ice — LC
- Ross seal (Ommatophoca rossii)Antarctica, drift ice — LC, rarely observed
- Crabeater seal (Lobodon carcinophagus)Antarctic drift-ice zone — LC, most numerous seal in the world
- Leopard seal (Hydrurga leptonyx)Antarctic marginal seas — LC, top predator
- Northern elephant seal (Mirounga angustirostris)Californian and Mexican coast — LC
- Southern elephant seal (Mirounga leonina)Subantarctic islands — LC, largest seal (up to 4 tonnes)
The crabeater seal, with an estimated population of seven to fifteen million animals, is probably the most numerous large mammal species on Earth after humans. It feeds on krill, not crab — the name is a nineteenth-century mistranslation. The southern elephant seal is the largest pinniped in the world: a fully grown male can weigh four tonnes and reach five metres in length. Next to those giants, the Dutch seals look almost modest.
Eared seals (Otariidae)
The eared seals number fourteen to sixteen species, divided between sea lions (five species, including the Californian, Steller's and Australian sea lion) and fur seals (nine to eleven species, including the northern fur seal, the Antarctic fur seal and the well-known South American fur seal). Strictly speaking, taxonomists reserve "true seals" for the Phocidae; the Dutch word zeehond is used more loosely. Eared seals do not occur in the Netherlands.
The walrus (Odobenidae)
The family Odobenidae today contains just one species: the walrus (Odobenus rosmarus), with two subspecies — the Atlantic and the Pacific walrus. Both live exclusively around Arctic pack ice. Male walruses can reach 1,500 kg and use their characteristic tusks to haul themselves onto the ice and to fight out social rank. Vagrants rarely turn up further south, but in 2022 the walrus "Freya" — in fact a female — caused a stir in Norwegian harbour towns.
Where do the Dutch species fit in?
Our common seal is one of the most successful and widespread seal species in the world; the Netherlands is just one of many core areas. The grey seal is much rarer globally — the British population accounts for more than forty percent of the world total, and the Waddenzee is an important extension of that. Both species are true seals: hind-flipper-propelled divers without external ears, raising their young in a short nursing period on the sandbar.
Anyone interested in the body plan that lets seals dive hundreds of metres deep and stay submerged for long periods can read on at anatomy. That page explains how a layer of blubber, an unusually high blood supply, and contractible muscles combine into a natural "dive tank".